A nuclear fuel aggregate that is at a final stage of a nuclear fuel cycle and that has finished combustion and cannot be used any more is called a used nuclear fuel aggregate. The used nuclear fuel aggregate includes a high radioactive material such as FP and is therefore necessary to be thermally cooled. For this purpose, the used nuclear fuel aggregate is cooled in a cooling pit at a nuclear power plant during a predetermined period of time (3 to 6 months). Thereafter, the cooled used nuclear fuel aggregate is accommodated in a cask that is a shielding vessel, and the used nuclear fuel aggregate accommodated in the cask is carried to a reprocessing facility by a track and the like, and is stored there. For accommodating the used nuclear fuel aggregate into the cask, a holding element having a latticed cross section called a basket is used. Each used nuclear fuel aggregate is inserted into each of cells that are a plurality of accommodation spaces formed in the basket. With this arrangement, a proper holding force is secured for holding the used nuclear fuel aggregate against vibrations during the transportation.
As prior examples of such a cask, various kinds of casks have been disclosed in, for example, the “Nuclear Eye (in Japanese), Nikkan Kogyo Shuppan Production, issued on Apr. 1, 1998, and Japanese Patent Application Laid-open Publication No. 62-242725. A cask based on which the present invention has been made will be explained below. It should be noted that this cask is shown for the convenience of the explanation, and does not correspond to a publicly known or publicly used one.
FIG. 19 is a perspective view showing one example of a cask. FIG. 20 is a cross-sectional view of the cask cut along an axial direction of the cask shown in FIG. 19. A cask 500 is constructed of a cylindrical shell main body 501, resin 502 as a neutron shielding unit provided on an outer periphery of the shell main body 501, an external cylinder 503, a bottom section 504, and a lid section 505. The shell main body 501 and the bottom section 504 have been forged from carbon steel as γ-rays shielding units. The lid section 505 consists of a primary lid 506 and a secondary lid 507 made of stainless steel or the like. The shell main body 501 and the bottom section 504 are connected together by butt-welding. The primary lid 506 and the secondary lid 507 are fixed to the shell main body 501 by bolts made of stainless steel. A metal-made O-ring exists between the lid section 505 and the shell main body 501, thereby holding an airtight condition inside the used nuclear fuel aggregate.
Between the shell main body 501 and the external It cylinder 503, a plurality of internal fins 508 are provided for carrying out a thermal conduction. Copper is used for the internal fin 508 to increase the thermal conductivity. The resin 502 is injected in a fluid state into spaces formed by the internal fins 508, and is cooled and solidified afterward. The basket 509 has a structure of having 69 angular pipes 510 assembled in a bundle as shown in FIG. 19, and is inserted into a cavity 511 of the shell main body 501 in a constrained state.
The angular pipes 510 are made of aluminum alloy having neutron absorbing material (e.g. boron: B) mixed into it in order to avoid the used nuclear fuel aggregate from reaching a criticality. On both sides of a cask main body 512, trunnions 513 (one is not shown in the drawing) are provided for suspending the cask 500. Further, on both ends of the cask main body 512, there are installed buffers 514 (one is not shown in the drawing) that have wood built inside thereof as a buffering material.
When actually manufacturing the cask 500, it is usually necessary to investigate on design conditions such as the number of used nuclear fuel aggregates, and their sizes and weights, etc. Specifically, it is preferable that the cask can accommodate a large number of used nuclear fuel aggregates, has a small external diameter, and has lightweight. However, according to the structure of the above cask 500, as the angular pipes 510 are in line contact with the inner surface of the cavity 511 at the outermost periphery, a space area S is formed between the basket 509 and the cavity 511. Therefore, the thermal conduction from cells 515 to the shell main body 501 cannot be carried out efficiently. Further, as the diameter of the shell main body 501 becomes large because of the existence of the space area S, the cask 500 has a heavy weight.
On the other hand, the volume of radiation that is leaked to the outside of the cask is prescribed by the total volume of neutron and γ-rays. Therefore, in order to reduce the weight of the cask 500, the thickness of the shell main body 501 maybe made smaller. However, because of the γ-rays shielding unit, the cask is required to have a thickness that is sufficient enough to secure a γ-rays shielding function at the shell main body 501 side. While the cask 500 can accommodate the unconventional number of 69 fuel assemblies, this number of accommodating the used nuclear fuel aggregates is reduced when the diameter of the shell main body 501 is made smaller in the structure to accommodate the used nuclear fuel aggregates within a predetermined weight.
It is an object of the present invention to provide a cask that has any one of the following. That is, improved thermal conductivity, higher accommodation capacity, compact size, and light weight.